The invention pertains to the field of inkjet printing and, in particular, to multiple inkjet cartridge array heads.
The operating principle of inkjet printheads is based on the ejection of a droplet of ink through a nozzle and onto a recording medium, such as a sheet of paper. The sheet of paper may or may not be specially treated, depending on the ink used and print quality desired. By arranging a plurality of nozzles in a pattern, such as a one- or two-dimensional array, characters or other images may be printed on the paper as the printhead is moved relative to the paper. This is achieved by appropriately sequencing the ejection of ink from the individual nozzles. The paper is typically moved each time the printhead has moved across the paper. The printhead is usually part of a disposable inkjet printer cartridge. To this end, commercial inkjet printer cartridges are designed for easy installation in, and removal from, the printer. The inkjet printer cartridge may contain a reservoir of ink, or the ink supply may also be external to the cartridge.
A typical inkjet cartridge comprises a housing, usually fabricated from plastic, and a nozzle plate, which is sometimes integrated with the nozzle actuators. The actuators are typically thermal or piezoelectric. An ink reservoir may be placed in the housing or ink may be fed to the housing via tubes.
With the advent of multiple inkjet head array products, there is a renewed drive to ensure that the inkjet nozzles of the different inkjet heads constituting the arrays are mutually aligned. Another example is color printing. In the case of color inkjet printing the relative positioning of the different colors of ink is very important, the human eye being extremely capable at detecting consistent deviations in ink dot positions. To this end, much effort has been devoted to ensuring that the various ink dots are correctly positioned in commercial inkjet equipment. In particular, there is a much attention focused on developing so-called xe2x80x9cpage-widexe2x80x9d devices.
In these machines, the inkjet head array extends across the entire width of the medium upon which printing takes place. This requires little if any motion from the heads and the medium essentially only needs to be moved with respect to an essentially stationary inkjet head array. However, with the very large number of nozzles involved in such systems, the alignment requirement among the multitude of nozzles is extreme. With the nozzles being stationary, there are few solutions available for situations of lateral misalignment between nozzles.
At the same time, much progress has been made towards the commercial production of inkjet cartridges at affordable prices. This has been achieved largely through automation of the manufacturing process. There is therefore much emphasis on obtaining a method for making large inkjet nozzle arrays through the simple expedient of combining commercial inkjet cartridges.
This attractive option is severely complicated by the fact that these products are often made via mass production processes that do not lend themselves to mechanical accuracy. The techniques employed in the fabrication of the actual nozzle assemblies of the cartridges are thoroughly capable of rendering the accuracies required, and the alignment among individual nozzles on a given nozzle assembly is extremely accurate. However, the manufacturing steps for the cartridge housings, and the steps for joining the nozzle assemblies to these cartridge housings, are not of sufficient accuracy. Nor are the materials employed in the manufacture of the cartridge bodies generally chosen with such considerations in mind. As regards commercial inkjet cartridges, the systems designer is therefore confronted with a highly accurate nozzle assembly, containing mutually well-aligned and evenly spaced nozzles, affixed to a cartridge housing of substantially lesser accuracy made via a variety of mass production techniques.
To align the nozzle plates on inkjet printer cartridges in such a way that nozzle plates are positioned in substantially the same location on all the various print cartridges, the nozzle plates are typically glued in position on the inkjet printer cartridges relative to a molded-in plastic datum formed on the inkjet printer cartridge body itself. This alignment process has a significant drawback in that the glue curing process causes nozzle plate to slightly shift as the glue is being cured. In addition, molded-in stresses in the plastic cartridge body creep during the thermal curing process.
Since this movement is substantially unpredictable, this alignment and gluing process can only produce print cartridges of which the nozzle plates are mutually registered to an accuracy of xc2x135 microns, even if the various cartridges are accurately positioned with respect to one another. This is significantly less accurate than what is required for the positioning of nozzles to provide a resolution of 1200 dots per inch from more than one cartridge and their associated sets of nozzles composed into a single array, as explained above.
Other, more elaborate techniques have been used to achieve higher alignment precision. One of these techniques automatically detects any misalignment of the nozzle plates once the print cartridges have been installed in a carriage, and then mechanically adjusts the positions of the print cartridges in the carriage. Using another relatively expensive method, an ink drop detector within the ink printer measures the location of a drop of ejected ink after being ejected from a nozzle, and a software algorithm compensates for any misalignment of the nozzle plates. Both of these techniques significantly increase the cost of the inkjet printer.
More precise alignment between two or more nozzle plates affixed to print cartridges installed in a single carriage has also been addressed by machining away datum projections on each print cartridge after its nozzle plate has been permanently secured to the print cartridge. The machined datum projections on the print cartridge make contact with surfaces on the carriage when the print cartridge is installed in the carriage such that the dimensions of the datums affect the position of the cartridge, and hence the nozzle plate, within the carriage. The datums on the print cartridge body are machined with reference to targets on the nozzle plate itself so that only rough alignment of the nozzle plate on the pre-machined print cartridge is required. The main disadvantage of this method is that any machining done on the finished cartridge produces debris which may block the inkjet nozzles.
In this particular approach an optical sensor is used to detect a target mark (such as a hole) on the nozzle plate, after the nozzle plate has been securely affixed to the print cartridge and after any adhesive has been fully cured. A mechanical means is then used to precisely position the print cartridge so that the target mark on the nozzle plate is aligned with a reference target stored in a memory. A machining tool is then used to remove portions of the datum projections on the print cartridge to cause the print cartridge, when installed in a carriage, to support the nozzle plate in precisely the same position with respect to the carriage irrespective of any misalignment of the nozzle plate on the pre-machined print cartridge. The machining of the datums may be made to such accuracy, that the overall alignment of the nozzle plates on multiple print cartridges, when installed in the carriage, will have been improved to an accuracy of better than 25 microns.
This technique whilst improving the accuracy in alignment, requires extensive intervention in the manufacturing process of the actual inkjet cartridges and adds further steps to the manufacturing increasing the cost and putting yield at risk. Given the fact that the cartridges are manufactured with the very intention of being disposable, like manufacturing steps of these items need to be kept to an utter minimum for both cost and yield reasons. It has the further drawback that it creates plastic machining dust into an environment where every precaution needs to be taken to avoid particulate continuation near the microscopic inkjet nozzles.
It is an objective of the present invention to provide a method that will allow mass-produced inkjet cartridges, produced to nominal tolerances via the minimum number of manufacturing steps, to be used to fabricate multiple cartridge inkjet arrays that have a high degree of alignment and registration between the nozzles from different cartridges.
A method is presented for aligning the nozzles of a number of individual inkjet cartridges that are combined to create a multiple inkjet head array for an inkjet printer. The method includes fabricating spatially referenced inkjet cartridge sub-assemblies by permanently attaching an intermediate fixture member to pre-made inkjet cartridges whilst referencing the intermediate fixture members to the inkjet nozzle arrays of the inkjet cartridges. The method further includes removably combining a number of such spatially referenced inkjet cartridge sub-assemblies in mutual spatial registration on a common fixture such that the inkjet nozzles of the different inkjet cartridges are all in spatial registration with one another. By this method an array of inkjet heads, of which all the nozzles are in the desired relative positions with respect to one another, is created. This method allows an array head to be maintained or repaired through the removal and substitution of an individual pre-registered inkjet cartridge sub-assembly.